1. Field of the Invention
This invention relates to a valve drive system for an engine in which radially disposed intake and exhaust valves are driven by intake and exhaust cam shafts.
2. Description of the Related Art
A system having four intake and exhaust valves radially disposed for each cylinder is known as an engine provided with a semi-spherical combustion chamber to improve combustion efficiency. In the valve drive system used for this type of engine, because the opening/closing directions of the intake and exhaust valves are different for each valve with respect to the direction perpendicular to the axes of the cam shafts, complicated structures are adopted to transmit the rotation force of the cam shafts to each valve (see JP-A-59-29709, for example).
The valve drive system disclosed in the patent publication is arranged in such a way that one cam shaft is supported in the center of a cylinder head for rotation. A cam surface of the cam shaft is formed parallel to the axial direction of the cam shaft, and two rocker arms are disposed for each intake or exhaust valve between the cam surface and the intake or exhaust valve.
A first rocker arm of one of the two rocker arms is supported for rocking movement on a first support shaft mounted parallel to the cam shaft, and has one end engaged with the cam surface and the other end extending toward the intake or exhaust valve. A second rocker arm is supported for rocking movement on a second support shaft mounted in a direction perpendicular to the axis of the intake or exhaust valve, and has an underside of the rocking end in contact with the intake or exhaust valve. The opposite side (top surface) to the intake or exhaust valve at the rocking end is engaged with the other end of the first rocker arm. That is, the valve drive system is arranged in such a way that movement of the cam surface is converted, by two rocker arms, to movement in a direction parallel to the axis of the intake or exhaust valve.
However, the valve drive system described above has problems of high manufacturing costs and large size because the number of the rocker arms used is high. In order to construct one rocker arm for one intake or exhaust valve to reduce the costs and downsize the system, it is contemplated that the system is arranged in such a way that the cam surface is inclined perpendicular to the axial direction of the intake or exhaust valve, thereby forming a three-dimensional cam and is engaged with the second rocker arm in a sliding relationship.
However, in implementation of this system, a problem arises in lubrication of the contact portion between the three-dimensional cam and the rocker arm. The lubrication of the contact portion is achieved by an oil film of lubricating oil formed between the cam surface of the three-dimensional cam and the sliding surface of the rocker arm. It is well known that the oil film is maintained when the foregoing two components are in line contact, but is broken when they are in point contact.
When the three-dimensional cam is manufactured as an industrial product, the contact state between the cam surface and the foregoing sliding surface tends to be in point contact due to manufacturing defects of the cam surface, and breakage of the oil film leads to wear of the sliding portion. Forming a highly accurate three-dimensional cam surface requires very long grinding work hours, resulting in a significant cost increase.
In order to solve the foregoing problems, an objective of the present invention is to provide a valve drive system capable of utilizing three-dimensional cams without decreasing lubricating capacity or increasing the cost and capable of effecting a cost reduction and downsizing the system by decreasing the number of rocker arms compared with the conventional valve drive systems.
Further, a conventional three-dimensional cam is used as described above, in order that the three-dimensional cam and the rocker arm slipper are in line contact with each other. However, the contact portions of both components must be precision-machined using special grinding machines, raising problems of longer processing time and higher processing cost.
In view of the above, an objective of another embodiment of the present invention problems is to provide a valve driving mechanism for the internal combustion engine that makes it possible to realize the line contact between the intake and exhaust cams and the rocker arms without requiring high precision machining so that friction and heat generation on the sliding surfaces of both components are restricted.
Furthermore, in a conventional sports type, i.e., high revolution engines with a small angle between valve axes and a large angle between the intake and the exhaust passage axes, it is difficult to dispose rocker arms around the cylinder center. That is, in a constitution in which a common rocker shaft passes through a rocker shaft hole bored across multiple cylinders, the rocker shaft hole will end up in intersecting the plug hole. Further, it is impossible to make by machining a long, small-diameter rocker shaft hole while maintaining a high precision of parallelism between the rocker shaft hole and the camshaft.
On the other hand, when a constitution is employed in which the rocker shaft is disposed outside the camshaft, arrangement of the intake and exhaust passages inevitably becomes disadvantageous.
In view of the above, an objective of yet another embodiment of the present invention is to provide a valve driving mechanism for a multi-cylinder engine which makes it possible to employ an integral type of cylinder head while disposing rocker shafts between intake and exhaust camshafts, and to increase rigidity of supporting the rocker arms by supporting them within a compact arrangement.